1. Field of the Invention
This invention relates to organosiloxane compositions curable by a platinum catalyzed hydrosilylation reaction to yield gels. More particularly, this invention relates to organosiloxane compositions that cure to yield soft gels exhibiting levels of dielectric properties desired for encapsulation of electronic devices in addition to a low level of extractable materials. Preferred compositions contain fluorinated alkyl radicals as substituents on a majority of the silicon atoms.
2. Description of Relevant Art
Polyorganosiloxane compositions that cure using a platinum catalyzed hydrosilylation reaction to form gels are known. Organosiloxane gels are described in U.S. Pat. No. 3,020,260, issued to Nelson on Feb. 6, 1962, and in U.S. Pat. No. 4,719,275, issued to Benditt and Maxson on Jan. 12, 1988. The gels described by Nelson are obtained by reacting an organosiloxane containing two silicon-bonded hydrogen atoms per molecule with a vinyl-containing copolymer wherein the repeating units are of the formulae RViSiO, R.sub.2 SiO, and CH.sub.3 R.sub.2 SiO.sub.0.5, R is methyl or phenyl and Vi represents vinyl.
The patent issued to Benditt and Maxson teach reacting (1) a polydiorganosiloxane containing fluorinated hydrocarbon radicals bonded to the silicon atoms together with at least two vinyl or other ethylenically unsaturated radicals per molecule with (2) an organohydrogensiloxane containing at least three silicon-bonded hydrogen atoms per molecule.
One method for reducing the hardness of cured organosiloxane gels prepared by a hydrosilylation reaction is to provide in the curable composition a molar excess of either silicon-bonded hydrogen atoms or ethylenically unsaturated hydrocarbon radicals such as vinyl. This results in relatively large amounts of unreacted liquid material in the cured gel that can be easily extracted from the cured gel. The presence of this liquid material can adversely affect both the heat stability of the gel and its resistance to degradation by the liquid hydrocarbons present in fuel and hydraulic fluids.
A second method for reducing the hardness of cured organosiloxane gels is to include in the curable composition (1) an organopolysiloxane containing two vinyl or other ethylenically unsaturated hydrocarbon radicals and (2) two types of organohydrogensiloxanes, one of which contains an average of at least three silicon-bonded hydrogen atoms per molecule, referred to as a crosslinking agent. The second type of organohydrogensiloxane contains two silicon bonded hydrogen atoms and is referred to as a chain extender. The relative amounts of silicon-bonded hydrogen present in the crosslinker and the chain extender determines the degree of crosslinking, which, in turn is one factor determining the hardness of the cured gel.
U.S. Pat. No. 4,340,709, which issued to Jeram et al. on Jul. 20, 1982 described organosiloxane compositions curable by a platinum-catalyzed hydrosilylation reaction. The composition contains a polydiorganosiloxane with vinyl radicals located only at the two terminal positions and the two types of organohydrogensiloxanes referred to in the preceding paragraph. The chain extender, referred to in this patent as a "linear hydride coupler", and the crosslinker each contribute at least 10 percent of the total silicon-bonded hydrogen atoms present in the curable composition. Jeram et al. disclose compositions containing from 4 to 19.4 weight percent, based on the weight of vinyl-containing polydiorganosiloxane, of linear hydride coupler.
Organosiloxane gels exhibiting reduced leakage of liquid organosiloxanes are described in U.S. Pat. No. 4,100,627, which issued to Brill on Jul. 18, 1978. The organosiloxane compositions used to prepare this gel comprise a dimethylvinylsiloxy-terminated polydimethylsiloxane exhibiting a viscosity of from 0.1 to 6 Pa.s, a dimethylhydrogensiloxy-terminated polydimethylsiloxane (referred to in the present specification as a chain extender) with a viscosity of less than 0.5 Pa.s, a polymethylsiloxane having at least three silicon-bonded hydrogen atoms and a hydrosilylation catalyst. The amount of dimethylhydrogensiloxy-terminated polymer is sufficient to impart a viscosity of from 4.5 to 30 Pa.s. The curable compositions exemplified in this patent contain from 97.5 to 99.9 parts by weight of the vinyl-terminated polymer and from 1.7 to 3.3 parts by weight of the chain extender. The effective viscosity of the chain-extended polymer ranged from 4.6 to 13 Pa.s.
Organosiloxane compositions that cure by reaction of a dimethylvinylsiloxy-terminated dimethylpolysiloxane with the combination of chain extender and a crosslinking agent, both of which contain methyl as the only silicon-bonded hydrocarbon radical, are described by Kroupa in U.S. Pat. No. 4,535,141, which issued on Aug. 13, 1985. The curable compositions also contain a resinous polyorganosiloxane as a reinforcing agent.
From 85 to 95 percent of the silicon-bonded hydrogen atoms in the curable compositions described by Kroupa are present in the organohydrogensiloxane containing silicon-bonded hydrogen atoms only at the terminal positions. In addition, the molar ratio of silicon-bonded hydrogen atoms contained in this ingredient to the vinyl radicals present in the curable composition is at least 1.6. The gels produced using the curable compositions described in the Kroupa et al. patent are relatively hard, tough, resilient and can be fabricated into optical waveguides such as touch sensitive screens used as input devices for computers. The compressability of organosiloxane gels is typically measured by applying a weighted load in the form of a conical or cylindrical foot to the surface of the gel. The weight required to compress a surface of a cured gel a distance of 0.23 millimeter is typically from 14.5 to 49 grams. Gels of this type are too hard for use as potting or encapsulating materials for electronic components such as transistors and integrated circuits.
One objective of this invention is to provide cured organosiloxane gels of sufficient softness for use as a potting and encapsulant material but which do not contain significant concentrations of unreacted organosiloxanes.
A second objective is to improve the dielectric properties, particularly dielectric constant and dissipation factor, of preferred organosiloxane gels containing methyl and 3,3,3-trifluoropropyl as the organic groups bonded to silicon.
The present inventor discovered that these objectives can be achieved by increasing the relative concentration of the diorganohydrogensiloxy terminated polydiorganosiloxane referred to hereinafter as a "chain extender" above the limits taught by the prior art. The chain extender is present in combination with an organohydrogenpolysiloxane containing an average of at least three silicon-bonded hydrogen atoms that serves as the crosslinking agent. Alternatively, the crosslinking agent can be a polydiorganosiloxane containing an average of at least three alkenyl radicals per molecule.